1. Field of the Invention
The present invention relates in general to adsorption separation processes. More particularly, this invention relates to fixed bed adsorption systems comprising multiple beds, and the integration of typical refinery streams as regenerant streams.
2. Description of the Prior Art
A variety of arrangements are known for adsorption separation processes. One type as practiced in the prior art, is shown in U.S. Pat. Nos. 4,734,199 and 4,740,631 and discloses as many as six distinct steps, namely:
(a) adsorption of one or more components from a feedstock mixture; PA1 (b) draining the bed of unprocessed feedstock; PA1 (c) regeneration of the bed using a heated purge fluid; PA1 (d) cooling down the newly regenerated bed in preparation for a new adsorption step by passage into the bed of a cooling medium; PA1 (e) draining the cooling medium from the cooled bed; and PA1 (f) filling the void space of the cooled bed with fresh feedstock.
These steps are more fully characterized below:
Adsorption step (a): During this step the liquid phase feedstock containing the impurities to be removed is passed through a vessel containing a suitable particulate adsorbent such as a zeolitic molecular sieve. As the feed passes through the adsorbent bed the impurities (sorbates) are selectively held up by the adsorbent. The feed, now containing significantly less impurity, leaves the adsorption vessel as product. The adsorption step is continued for a fixed time interval or until the impurity levels in the product exceed specifications. At this time the feed is directed to another adsorption vessel of the system, this vessel having been previously regenerated.
Feedstock Drain Step (b): During this drain step, the feedstock remaining in the void space of the vessel at the end of the adsorption step (a) is drained by gravity or pumped out and recycled to feed. If the vessel is drained slowly then the time required for draining will constitute a significant portion of the overall cycle time. If the vessel is drained quickly then the additional flow rate due to material combining with the feed must be considered when sizing the sorbent requirement. In either case, the elimination of the drain step would be of considerable advantage in a liquid phase sorption system.
Regeneration Heat Step (d): After draining step (b), a heated regeneration medium is passed through the adsorbent bed. As the adsorbent is heated it releases the previously sorbed sorbate. The sorbate passes into the regeneration heating medium and is carried out of the system by the latter. The heating step is continued until the bulk of the impurities have been carried out of the adsorption vessel. Regeneration heating is usually carried out with a regenerating medium differing from both product and feedstock.
Regeneration-Cool Step (d): During this step a cooling medium is passes through the hot adsorption vessel to carry out the sensible heat remaining in the adsorption vessel at the end of the regeneration heat step. The cooling is continued until the bulk of sensible heat is carried out of the sorption vessel. In many instances cooling is carried out with a medium other than the feedstock. It is customary to drain this medium before proceeding to the fill step. This adds another step to the overall process cycle.
Cooling Medium Drain Step (e) the step in which the cooling medium remaining in the adsorbent bed void space at the end of Regeneration Cool Step (d) is removed from the bed either by gravity flow or by pumping.
Void Space Filling Step (f): During the fill step, either product or feedstock is used to fill the void spaces in the adsorption vessel before returning the vessel back into service. This is necessary since failure to do so will result in two phase flow and vapor lock. In large volume sorption vessels the time required for filling the vessel can be substantial especially since often the rate at which feed or product is available is often limited. Upon completion of the fill step the sorption vessel is ready to be put back into the sorption step.
In the above processes the regenerant fluid, although heated, remains in the liquid phase requiring a drain step at the end of the regeneration-cool step. It is preferred to operate the process with the regenerant in the vapor phase during the desorption step. Operation of the regeneration cycle in the vapor phase permits the processing of feedstocks with relatively small quantities of oxygen-containing compounds. The objective of the present invention is to remove trace amounts of oxygenates such as methanol, methyl tertiary butyl ether (MTBE), dimethyl ether (DME), tertiary butyl alcohol (TBA) and water from a reactor effluent stream wherein the concentration of each of these oxygenates ranges from 20 to 2000 ppm wt. and the total amount of oxygenates in the stream ranges from 1000 to 2500 ppm wt. The operation of the regeneration in the vapor phase further permits a pressure assisted drain step to drain the liquid feedstock from the bed at the beginning of the regeneration cycle. A small amount of vaporized regenerant less than 20% of the total is permitted to enter the effluent end of the adsorber bed, forcing the feedstock from the bed. This operation significantly shortens the drain step and provides some initial bed heating. When all of the feedstock has drained from the bed, the full flow of vapor regenerant can be passed over the adsorbent to desorb the oxygen-containing hydrocarbons. The vapor is recirculated and heated until the bed reaches the required temperature for desorption, typically this ranges from 200.degree.-300.degree. C. At the conclusion of the desorption step, the bed must be cooled to adsorption conditions, typically ranging from 25.degree.-50.degree. C. Typically, the bed is cooled by introducing a liquid regenerant which may be the feedstock, the product, or a separate fluid. The initial passing of liquid through the bed in an upflow manner often results in a degree of vaporization of the regenerant liquid which provides further cooling.
The ideal regenerant is a dry, sulfur-free gas. However, in a petroleum refinery there are very few sources of dry gases with a minimum of impurities such as sulfur compounds which would be suitable for this application. Impurities such as water, sulfur and heavy hydrocarbons may contaminate the adsorbent and reduce its effectiveness or shorten its useful life.
Typically, this process used lighter molecular species or the same molecular species as the product for the regenerant. It was generally believed, by those skilled in the art, that regenerant streams containing hydrocarbons that are heavier than the product would interfere with the operation of the adsorbent. Normal butane was often used for the regenerant. When this butane could be blended into gasoline, there were some gasoline octane benefits. However, current U.S. Environmental Protection Agency requirements to reduce the vapor pressure of gasoline has restricted this use for butane.
One of the major applications for this technology is in the manufacture of a high octane motor gasoline component such as methyl tertiary alkyl ethers in these processing arrangements as described in U.S. Pat. No. 4,816,607. The production of ethers by the reaction of an isoolefin with an alcohol is well known and is practiced commercially. This highly selective reaction is also used to remove olefins, especially isobutylene, from mixed hydrocarbon streams such as the C.sub.4 streams produced in steam cracking plants which produce ethylene. Increased attention has been focused on ether production due to the rapidly increasing demand for lead-free octane boosters for gasoline such as MTBE. A detailed description of the processes, including the catalysts, processing conditions and product recovery, for the production of MTBE from isobutylene and methanol are provided in U.S. Pat. Nos. 2,720,547 and 4,219,678 and in an article at page 35 of the Jun. 25, 1979 edition of Chemical and Engineering News. The preferred process is described in a paper presented at the American Institute of Chemical Engineers 85th National Meeting on Jun. 4-8, 1978 by F. Obenaus, et al. Descriptions of integrated processes, including those which utilize butane isomerization are found in U.S. Pat. Nos. 3,726,942, 4,118,425, 4,252,541, and 4,329,516.
In U.S. Pat. No. 4,814,517 to Trubac a dual or compound adsorption bed containing silica gel and zeolite 13X is employed to first selectively remove methanol and then selectively remove dimethylether from an etherification effluent within a process scheme for the production of methyl tertiary butyl ether, MTBE. The adsorber system is regenerated in the liquid phase with normal butane as the regenerant.